1. Field of the Invention
This invention pertains generally to mixed signal integrated circuit board applications, and more particularly to an integrated circuit board with a metal substrate with excellent crosstalk isolation capabilities, enhanced heat sinking and true ground.
2. Description of Related Art
The rapidly growing markets for mobile communications and wireless networking have created a demand for high-performance mixed-signal radio frequency integrated circuits (MSIC). Such circuits are typically composed of a combination of metal oxide semiconductor (MOS) digital circuits with analog circuit elements including passive components such as inductors on a single chip. These circuits have the advantage of smaller form factor, higher packing density and lower cost.
However, the integration of high performance analog circuits with noisy digital circuits at high frequencies is often limited or hampered by the presence of digital noise called “crosstalk” that inevitably interferes with analog circuit function. For example, noise from switching transients in digital circuits can travel through the silicon substrate and degrade the function of sensitive circuit elements such as low noise amplifiers or phase-lock loops. Integrated inductors and other circuit elements can act as antennas that receive spurious signals and other substrate noise from other sections of the substrate.
Suppression of substrate noise is critical to system-on-chip (SOC) feasibility and design. A number of approaches have been attempted to isolate sensitive elements from crosstalk interference. High resistivity silicon, junction isolated wells, silicon-on-insulator (SOI) wafers, guard rings, and micromachined structures have been used to suppress crosstalk through the substrate with limited success. In addition, bulk machining, guard rings and high resistivity silicon is costly and may degrade during exposure to high temperatures. At high frequencies, the limited oxide thickness of SOI substrates that are required to avoid structural failure due to thermal expansion mismatch cannot provide sufficient RF crosstalk isolation due to the inverse dependence of the impedance of capacitive channels on the frequency and the fact that high impedance, in addition to high resistance, is necessary to isolate radio frequency crosstalk.
Accordingly, there is a need for a substrate structure that will minimize substrate loss for radio frequency or microwave passive components, coplanar wave guide (CPW) lines, and crosstalk between the digital, analog, RF or microwave circuit blocks via the substrate. The present invention satisfies this need, as well as others, and generally overcomes the deficiencies of the prior art.